1. Field of the Invention
This invention relates to methods and apparatuses for liquefying natural gas. In another aspect, the invention concerns a liquefied natural gas (LNG) facility employing a dual-refluxed heavies removal column.
2. Description of the Prior Art
Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form.
Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers.
Storing natural gas in its liquefied form can help balance out periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream via indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, acid gases, nitrogen, and ethane and heavier components) from the natural gas stream at different points during the liquefaction process.
At some point during the liquefaction process, many LNG facilities employ one or more distillation columns operable to remove a majority of the butane and heavier components from the natural gas stream. Failure to remove these heavy components prior to the complete liquefaction of the natural gas will cause the higher molecular weight materials to freeze and plug downstream heat exchangers and other process equipment. In most cases, ensuring adequate heavy hydrocarbon removal from the natural gas stream is complicated by the need to maximize operating pressure of the distillation column or columns in order to minimize horsepower requirements for the facility's compressor/driver systems, which are typically the largest single energy consumers. As the operating pressure of the column or columns nears the critical pressure of methane (i.e., about 550 psia), the column's separation efficiency declines rapidly, resulting in increased carryover of butane and heavier material into downstream equipment. Alternatively, operating the column at a reduced pressure in order to avoid heavies carryover increases energy consumption and, ultimately, results in higher plant operating costs.
Thus, a need exists for an LNG facility capable of minimizing compressor/driver horsepower requirements while efficiently separating the heavy hydrocarbon material from the natural gas stream.